Product focus: CMEMS oscillator architecture

The following is a white paper by Silicon Labs with an innovative new process and technology that I believe deservessome level of detail and explanation for informative and educational purposes for EDN readers. Learning about this technology will help all designers give birth to new ideas and architectures as well as help those other designers to effectively integrate this type of product into their systems.

Introduction

CMEMS® technology is an innovative CMOS + MEMS manufacturing process developed by Silicon Labs, a leading supplier of timing solutions. The term CMEMS is a contraction of the acronyms CMOS and MEMS (microelectromechanical systems). CMEMS technology offers many benefits over traditional oscillator approaches, ranging from scalability, customer-specific programmability and 0-day samples, to long-term reliability and performance. This white paper describes CMEMS process technology, existing hybrid oscillator architectures and the Si501/2/3/4 (Si50x) CMEMS oscillator architecture.

A Brief Overview of Crystal Oscillators

The $3 billion (USD) per year frequency control market has been dominated by quartz crystals and quartz-based oscillators for many decades. Almost every type of electronic equipment relies in some manner on a tiny piece of machined quartz rock to generate at least one of many potential operating frequencies. Figure 1 shows a classic quartz-based oscillator and its components.

Figure 1. Crystal Oscillator Components

Over the last few decades, quartz manufacturing has reached new levels of sophistication, delivering smaller, thinner and higher-frequency solutions. While these manufacturing advances are important and certainly measurable, the overall process in terms of the required processing steps has not changed very much.

The manufacturing process starts with a blank piece of quartz that must be cut, lapped, polished, plated and further manipulated to achieve its required output frequency. Beyond this initial set of coarse steps, the process continues to refine the quartz crystal to achieve the required specifications. At each step, there are opportunities for the various components to suffer yield fallout. The complete system is not tested for yield and performance until final packaging, when the quartz crystal is hermetically sealed in a ceramic package with the silicon amplifier. Figure 2 provides a high-level overview of the enormously complex quartz crystal manufacturing process, which supports hundreds – if not thousands – of unique crystal shapes and cuts for specific frequencies for target systems.

Figure 2. Crystal Oscillator Production Steps

In 2004, Silicon Labs introduced its family of crystal oscillators (XOs), which leverages innovative and proprietary mixed-signal expertise to generate almost any frequency from a single, crystal-based reference frequency. This revolutionary technology, called DSPLL®, achieves performance similar to that of the highest-performing crystal-based oscillators, but it obviates the requirement for unique crystals for different frequencies, thereby minimizing a very large part of the quartz manufacturing process, including unique cuts and plating. This approach shortens the time from receiving an order to delivering samples from many weeks to less than two. It also removes risk from the supply chain by using a single, high-volume crystal frequency. DSPLL-based XOs have been broadly adopted in the electronics industry and today represent a substantial business for Silicon Labs.

With CMEMS, Silicon Labs has again taken an important technological step forward in the frequency control market. CMEMS replaces the crystal resonating element in XOs with micromachined semiconductor resonators. CMEMS is a proven technology that enables high-performance MEMS to be constructed directly on standard, advanced-process-node CMOS wafers (<180 nm). Combined with Silicon Labs’ mixed-signal expertise, CMEMS technology enables a single reference frequency to be used to generate almost any required frequency output that is as stable as mass-produced, high-volume quartz-based oscillators.

The Si50x CMEMS oscillator family is the first product offering to use CMEMS technology. It is optimized to support high-volume, low-cost applications where power and size are important, such as in the industrial, embedded and consumer electronics markets. Upcoming CMEMS products will be introduced to support other higher-performance markets as well as applications with unique requirements, such as ultra-low-power and multiple simultaneous frequencies.